Sep. 8, 2008 Janet S. Sunness, M.D., Director of the Hoover Low Vision Services at the Greater Baltimore Medical Center, and her fellow researchers have discovered a simple and inexpensive way to predict the rapid loss of visual acuity, the ability to see detail, in “dry” AMD patients.
Of patients with GA who have visual acuity of 20/50 or better, about one-quarter become legally blind within four years. Other GA patients never progress to severe vision loss. The ability to identify GA patients at highest risk is important to patient care as well as future AMD research. Dr. Sunness’ study evaluated GA patients who began the study with 20/50 or better vision; their performance under “low luminance,” or low-light conditions, strongly predicted whether they would suffer significant loss of visual acuity within two years. This study was the first to quantify predictive values for “low luminance visual acuity deficit” and describe the importance of this predictive tool for GA patient screening.
It is known that the visual function of GA patients is significantly worse in low-light conditions. In annual exams over the two-year study period, Dr. Sunness’ team measured the patient’s standard visual acuity, the ability to read lines on a chart under regular testing conditions, and then repeated this measurement with the patient wearing moderate gray sunglasses (2 log unit neutral density filter). On average, elderly patients without GA were able to see two fewer lines with the filter in place, while GA patients with 20/50 vision or better at baseline saw five fewer lines. The 91 study participants were a cohort of a larger 1992 to 2000 National Institutes of Health-funded study, directed by Dr. Sunness, of the natural history of GA progression at the Wilmer Eye Institute at Johns Hopkins School of Medicine.
The participants with the worst “low-luminance deficit” (LLD) at baseline were significantly more likely to have lost visual acuity by the end of the study than the better LLD group. Contrast sensitivity and reading speed were also measured for participants; the retinas were photographed; the eyes were examined for AMD progression; and the total area of GA in each eye was determined. Another predictor of visual acuity loss was a significantly reduced reading rate at baseline. Factors that did not predict visual acuity loss in these patients included: age, gender, the GA total area at baseline, or the rate of GA enlargement during the study period.
GA is a public health issue that deserves heightened attention and research, Dr. Sunness says, as the disorder causes more severe vision loss than diabetic macular edema, a well-recognized public health problem, and as treatment or prevention of GA is not currently available. Also, GA affects both eyes for most patients, resulting in vision loss that is more devastating than when eye disorders affect one eye only.
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